Forwarding Mail without opening it
 

If I forward all my Fastmail mail and not open it in F/m is it checked for virus's etc before being forwarded.

I believe that depends on how you forward it. To recap,
1) You can specify external account(s) as the recipients when you set up an alias. When you do this, the Fastmail MX servers will redirect the messages targeting that alias after minimal processing, and without going through virus checks.
2) Alternatively, you can forward messages using the sieve script (or, more commonly, using rules to generate the appropriate sieve code). When you do this, the message has passed through the virus checks before hitting the sieve script.

Thank you, I like the sound of the sieve script option.

The advantage of targeting external addresses in aliases is that it works when most of Fastmail's servers are down. It only requires the main or alternative MX servers to be running. That makes it the way to go for backup accounts designed to provide an alternative place to read new mail when Fastmail is mostly down for a significant period. About a decade ago, that was important. Today, for most people Fastmail's availability is sufficiently good that robust backup accounts are not critical.